Shower problem when I turn my hot water tap in my bathroom vanity on my shower comes on. What the!!!!!

You need a new cartridge for the shower located behind the handle.

Type of water heater is unknown
If you have single-handled faucet the faucet cartridge might be bad.
If you have recirculation system, then check valve might be bad.
Copy following links for troubleshoot.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

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As per my view, you should immediately contact to a professional who can fix this shower problem very easily and quickly. You can also explore latest shower enclosures that are available at reasonable price.

You need to have the flow switch looked at. The low flow of the water to the lavs and the kitchen sink, they all have as small demand next to t bath or shower,
Tom

Check the water supply lines to the shower at the faucet handles. If the fitting for the hot water line connected to the handle is loose, it can twist and kink off the water supply further down the line when you turn the faucet handle. I'm not exactly sure how this could be relevant otherwise, but under normal design circumstances this type of scenario with your shower wouldn't matter for other faucets.

the problem is the hot/cold mix is set wrong ---
remove the water control handle to expose the mix valve -- you should see a small set screw that will either increase or decrease the travel of the hot water valve. --- sometimes instead of a set screw there is a little pop off plastic set ring,, pop off and set to a more advanced travel.
this will increase the amount of hot water that flows to the shower head and the bath tap.

c...

The tap fitting on the shower may have failed. Some of the modern fittings have a more complicated internal mechanism than the traditional taps and can fail rendering the tap inoperable. Turn off the water at the mains. First take off the tap handle and then with a spanner carefully unwind the tap spindle. Be very carefull not to use excessive force in case you damage the pipework behind. If it does not come ralatively easily leave it for someone else. When you have this out have someone crack the main valve and you should have hot water coming out. If the fitting itself is the problem you may be able to take the spindle and washer out of a standard brass garden tap and use it as a temporary fix untill you get a propper replacement. Just another thought, you may have an isolation valve on the hot water line to the bathroom. Could a kid have turned it off?

water must be set at 3lts a min ,check diaphragm valve move when hot tap is on, check microswitch ,best of luck

There is a handle limit stop under the handle. Remove the handle with an allen wrench and you should see a plastic piece with a tab on it fit into the assembly. Remove it and rotate it slightly to allow more travel in the handle and reinsert. You will have to test it to see if it allows enough hot water to mix in. If it is still not warm enough repeat as needed. Once you have the water at a comfortable temperature reinstall the handle and enjoy.